Real-time wavefront correction system

ABSTRACT

An optical imaging system having the capability of detecting and eliminating in real-time phase distortions in a wavefront being imaged by the optical system. The resolution of ground based telescopes is severely limited by random wavefront phase changes and tilts produced by atmospheric turbulence. The disclosed invention was designed to overcome this problem. In the disclosed invention, an AC, lateral shearing interferometer measures in real-time the relative phase differences of the wavefront being imaged by the optical system. Phase differences measured by the shearing interferometer are directed to an analog data processor which, in combination with other circuitry, generates a plurality of electrical signals proportional to the required phase corrections at different areas of the wavefront. The electrical signals are applied to a phase corrector upon which the wavefront is incident to change the relative phase at various locations of the wavefront to achieve a wavefront in which the phase distortion is removed. In one embodiment the phase correction device consists of a mirror having an array of piezoelectric elements which function to selectively deform the mirror to correct phase distortions in the wavefront. In a second embodiment the phase correction device consists of a refractive device which has the capability of having its index of refraction selectively changed in different areas to correct phase distortions in the wavefront.

United States Patent 11 1 1111 3,923,400

Hardy Dec. 2, 1975 REAL-TIME WAVEFRONT CORRECTION [57] ABSTRACT SYSTEMAn optical imaging system having the capability of del l lnVenlOfI J y,LeXlngton, a tecting and eliminating in real-time phase distortions 3 Itt in a wavefront being imaged by the optical system.

[7 1 Asslgnee ek corporanon Lexmg on Mass The resolution of ground basedtelescopes is severely l l Flledi J 1974 limited by random wavefrontphase changes and tilts [21] Appl No: 430,456 produced by atmosphericturbulence. The disclosed 1nvent1on was designed to overcome thisproblem. In

the disclosed invention, an AC, lateral shearing inter- [52] US. Cl356/107; 356/111; 350/160 R ferometer measures in real-time the relativephase dif- [51] Int. Cl. G01B 9/02; G02B 5/23 ferences of the wavefrontbeing imaged by the Optical Field of Search 7, 9, HO, system. Phasedifferences measured by the shearing 356/1 1 350/160 299 interferometerare directed to an analog data processor which, in combination withother circuitry, generl l References Cited ates a plurality ofelectrical signals proportional to the UNITED STATES PATENTS requiredphase corrections at different areas of the 3,463,572 8/1969 Preston, Jr350/161 Wavefront The electrical Signals are applied 3527537 9 1970Hobmugh H 356/106 R phase corrector upon which the wavefront is incident3,632,214 1/1972 Chang et al 356/106 R to hang the relative phase atvarious locations of the 3,836,256 9/1974 Peters 356/109 wavefront toachieve a wavefront in which the phase Primary Examiner-Alfred E. SmithAssistant ExaminerConrad J. Clark Attorney, Agent, or FirmHomer 0.Blair; Robert L. Nathans; Gerald H. Glanzman TELESCOPE OBJECT/ l/E RAWF/ELD LE/VS l0 LE/VS distortion is removed. In one embodiment the phasecorrection device consists of a mirror having an array of piezoelectricelements which function to selectively deform the mirror to correctphase distortions in the wavefront. [n a second embodiment the phasecorrection device consists of a refractive device which has thecapability of having its index of refraction selectively changed indifferent areas to correct phase distortions in the wavefront.

12 Claims, 10 Drawing Figures PHASE REL/J) CORRE 07'0/1'" LEA/.5 l8

US. Patent Dec. 2, 1975 Sheet 3 Of4 3,923,400

DRWER TO PHASE fAMP AMP CORRECT OR II II N =1/4[A+B+c+ D+ 1-b-c+d1COMPUTl/VG ELEMENT US. Patent 1 PHASE SAMPLING AREA Sheet 4 of 4 OPT/MUMLOCAT/O/V X z 0 OF PHASE SENSORS a H6 85 SHEA/QED WA I/EFRO/VT MODELIMAGE SENSOR TELESCOPE CORRECTED PHASE OBJECT/V5 IMAGE CORRECTOR LEA/S70F/ELD ggflg DEFLECT/O/V LENS 7 ram:-

REL/l) $212 LE/VS 75 SCAN GENERATOR M/15K34 1 l PHASE DISTORTION AEKE' QOETEGTOR REAL-TIME WAVEFRONT CORRECTION SYSTEM BACKGROUND OF THEINVENTION The resolution of ground based optical imaging systems isseverely limited by random wavefront tilts and phase changes produced byatmospheric turbulence.

The resolution of such optical systems is usually limited to one or twoare seconds by the atmosphere and may be considerably improved if theatmospheric distortion is measured and then corrected. It is possible toperform these operations either prior to detection (predetection) bycorrecting the wavefront in real-time before recording the image on tapeor film, or alternatively to record the distorted data first and performthe processing in a subsequent operation (postdetection). A comparisonof the performance capabilities of predetection and postdetectionprocessing systems indicates that in practice predetection processingoffers superior performance.

The present invention relates generally to ground based telescopicsystems which view objects of interest through the atmosphere. Moreparticularly, the present invention relates to a new and improvedtelescopic optical system which has the capability of detecting andcorrecting in real-time phase distortions in the wavefrom being imagedby the optical system. More particularly, the present invention relatesto a system which senses the wavefront distortion produced by theatmosphere on a wavefront incident on the entrance pupil of thetelescope, and reimages the incident wavefront onto a phase correctiondevice which removes the distortion to produce a corrected image of theobject being observed. Because of the rapidly changing nature of theatmosphere, the system must provide a real-time response having aresponse time to changing conditions of no longer than a fewmilliseconds.

An AC shearing interferometer measures the relative phase shifts atpoints in the wavefront separated by the shear distances in the x and ydirections at an array of locations in the sheared image plane. Thecomputation required to convert the x and y phase values measured by theshearing interferometer into the phase correction values required by thepresent invention is mathematically a matrix inversion followed by aleast squares smoothing operation. According to prior art dataprocessing techniques, a relatively large digital computer would berequired to carry out this operation serially for a large number ofchannels at the required speed. Also, input and output multiplexers andA/D and D/A hardware would be required. When real-time operation isrequired, as in the present invention, serial operation is too slow,unless an extremely fast computer is utilized. The cost of such a priorart data processor system would be very high.

SUMMARY OF THE INVENTION In accordance with a preferred embodiment, asystem is disclosed for changing in real-time the phase relationship ofdifferent areas of a wavefront. The phase relationship of differentareas of the incident wavefront are detected, and the detected phaserelationships are utilized to apply signals to a phase corrector, whichhas the wavefront incident thereon, to change the phase relationships ofthe different areas to reshape the wavefront.

The use of real-time predetection processing allows random variationsproduced by the atmosphere to be largely removed, thus allowing theinherent resolution of the telescopic optical system to be utilized. Inprinciple, the wavefront correction device will also compensate fordistortion in the optical system itself so that near diffraction limitedperformance may be obtained from imperfect optical systems. Thepreferred embodiment makes use of an AC white light shearinginterferometer to measure the wavefront distortion. In the preferredembodiment, the output of the shearing interferometer is processed by ahigh speed parallel data processor to obtain the required phasecorrections.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a preferredembodiment of applicants invention in which the phase corrector is amirror which is selectively deformed by a plurality of piezoelectricelements.

FIG. 2 depicts the lateral shearing interferometer which is utilized inthe generation of required phase corrections for different areas of thewavefront.

FIG. 3 illustrates one phase detection channel at the output of theshearing interferometer.

FIG. 4 illustrates the manner in which phase shift vectors areselectively combined to determine the required phase shift for one areaof a wavefront.

FIG. 5 illustrates a complete matrix wherein phase shift values arederived for 21 separate areas of a wavefront.

FIG. 6 shows an analog circuit for carrying out the vector combinationsillustrated by FIGS. 4 and 5, and also circuitry leading to the phasecorrector.

FIG. 7 illustrates a second embodiment of an analog circuit for carryingout the vector combinations.

FIG. 8 illustrates some optimum relationships between photodetectors inthe photodetector array in the shearing interferometer and correctorpoints in the phase corrector.

FIG. 9 shows another preferred embodiment of applicants inventionwherein the phase corrector is a refractive element, the refractiveindex of which is selec' tively controlled by the incidence of lightfrom a cathode ray tube.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1 there isillustrated a preferred embodiment of applicants invention. In FIG. 1 anobjective lens 10 at the entrance pupil to the telescopic optical systemproduces a raw image 12 of a distant object at the prime focal plane ofthe optical system. This image may be severely distorted because ofrandom wavefront tilts and phase shifts produced by atmosphericturbulence. In FIG. 1 the telescope is illustrated as a refractiveinstrument. However, the principle of operation of applicants inventionworks equally well with reflective telescopic optical systems. A fieldlens 14 and a relay lens 16 function in combination to produce an imageof the wavefront received at the entrance aperture (at objective lens10) on a phase corrector 18. Also, the field lens 14 functions toprevent off axis radiation, illustrated at 20, from missing the relaylens 16. The image of the incoming wavefront produced on phase corrector18 allows it to selectively change the phase of different areas of thewavefront image.

In the embodiment of FIG. 1, the phase corrector 18 includes an array ofpiezoelectric elements having a mirrored surface 21. The mirroredsurface is selectively deformed in accordance with electrical signalsapplied to the array. A suitable phase corrector having this design isdescribed in detail in US. patent application Ser. No. 392,163 forMONOLITHIC PEIZOELECTRIC WAVEFRONT PHASE MODULATOR, filed Aug. 27, 1963.The wavefront is reflected off the mirrored surface 21 to a beamsplitter 24. Beam splitter 24 allows a first portion of the beam to passto a lens 26 which forms a corrected image of the object being viewed onthe surface of an image sensor 28. Image sensor 28 may be photographicfilm, or may be an electro-optical device such as an image tube. Beamsplitter 24 deflects a second portion of the beam to a lens system 29which forms an image of the reference object on a phase distortiondetector system 30. In the preferred embodiment the phase distortiondetector system 30 consists of an AC lateral shearing interferometer asdescribed in detail in US. patent application Ser. No. 346,365, forSHEARING INTERFEROMETER, filed Mar. 30, 1973, by James C. Wyant, and nowUS. Pat. No. 3,829,219.

Reference should be made to that patent application for a more completeunderstanding of the operation of the shearing interferometer. However,an abbreviated explanation of the interferometer will be given below.The net result of the optical arrangement of FIG. 1 is that an image ofthe reference object is formed at a focal plane 32 (see FIG. 2) in thephase distortion detector 30. This reference object may be a referencestar which is different from, but angularly close to, the object ofinterest, or it may be the object of interest itself. Alternatively, itmay be a laser beam angularly close to the object of interest directedfrom a vehicle through the atmosphere to the telescope. As shown inFIGS. 1 and 2, a mask 34 may be provided to allow only the image of thatone object to pass to the focal plane 32.

FIG. 2 illustrates the principle of operation of the interferometer. Twoslightly different diffraction gratings 38 and 40 are placed near thefocal plane 32. These diffraction gratings are slightly different toproduce two diffracted cones of light at two slightly different anglesand having a common region of overlap. In the illustrated embodiment thedifiraction gratings have line spacings at slightly differentfrequencies. The two diffraction gratings 38 and 40 may be producedholographically on a single photographic plate. For any given wavelengthof light, the two gratings produce two diffracted cones of rays at twoslightly different angles. This results in the formation of a shearinginterferogram in the region of overlap 42 between the two cones oflight. The two diffracted beams are incident upon a third achromatizingdiffraction grating 44 having a diffraction frequency halfway betweenthe diffraction frequencies of the gratings 38 and 40. As explained inthe patent application referenced above, the achromatizing diffractiongrating 44 allows the interferometer to operate with white light. Therays are diffracted again by grating 44 such that they are parallel tothe optical axis. This is true regardless of the wavelength of the lightbecause of the achromatizing grating 44. A field lens 46 produces animage at a further image plane 48 of the wavefront at the telescopeentrance pupil. Because of the two diffracted cones of light, the fieldlens 46 actually produces two images of the wavefront at the entrancepupil slightly displaced from each other, and a shearing interferogramis produced in the region of overlap between the two images. In theinterferogram the shear distance L Aa(f f where )t is the wavelength ofthe light, a is the distance between the image plane diffraction gratingand the pupil image plane and f and f; are the diffraction gratingfrequencies. Also, the fringe spacing S RA/L where R is the wavefrontradius. Substituting for L in the two equations, it is determined thatthe fringe spacing S is independent of A and is directly proportional tothe wavefront radius R. Measurement of the fringe spacing S willtherefore reveal R or the departure of the wavefront from its idealshape which would be plane if the reference source were at infinity.

In the preferred embodiment, a higher sensitivity is obtained bymodulating the interference pattern and sensing the phase at variouspoints in the pattern with an array 50 of photodetectors. As taught inthe abovereferenced patent application, the interference pattern may bemodulated by moving the gratings 38 and 40 with a velocity V in adirection normal to the line diffraction patterns on the gratings. Eachpoint in the interference pattern is modulated at a modulation frequencyf,, V(f f In one embodiment the diffraction gratings 38 and 40 are movedlinearly by an electromagnetic translating drive 41 similar to a movingcoil loudspeaker movement. In this embodiment, grating frequencies f andf of 400 and 440 cycles per mm are utilized, giving a differencefrequency (f f of 40 cycles per mm. If the time allowed for phasemeasurement is one millisecond and if it is desired to integrate thismeasurement over 10 cycles of modulating frequency to provide a betteraccuracy, then the required modulating frequency f,,, is 10 kHz, and theresultant grating velocity V is 250 mm/sec, which is easily achieved bythe moving coil loudspeaker movement 41.

As explained in the above-referenced patent application, the system asshown in FIG. 2 yields wavefront information in only one direction (X).Since the wavefront is two dimensional (X and Y), a two dimensionalsystem is required. This is achieved by utilizing two more diffractiongratings of the same type and in the same location as gratings 38 and 40but with the direction of the diffraction patterns running at relativeto the patterns of 38 and 40. In the preferred embodiment all fourdiffraction gratings are produced holographically on one photographicmedium. A second achromatizing grating, similar to grating 44 and havingthe same orientation as the third and fourth diffraction gratings willalso be required along with a second array of photodetectors to measurephase differences and derive wavefront information in the Y direction.The x and y measuring systems may be combined into one system byproducing all four diffraction gratings on one photographic medium andmoving the photographic medium in a direction 45 relative to bothdirections of the four diffraction patterns. The 45 movement will thencause the gratings to have a component of movement in each of the X andY directions. Referring to the calculation above, if the requiredgrating velocity of one set of gratings is 250 mm/sec, then displacementat 45 relative to both grating directions would require a drive velocityof V2 X 250 355 mm per second. Using a drive system with an excursion ofi2 mm, the required drive frequency is 28 Hz, which is easily achievablewith the moving coil loudspeaker movement 41.

Because of the rapidly changing nature of the wavefront distortion,parallel output sensor arrays are preferred over sequential scanningsystems. A suitable array for this application would be an imageintensifiersilicon diode array having a separate electrical output foreach diode. This arrangement is feasible because of the moderate numberof diodes in each array. In the disclosed embodiment an array of 32photodiodes is required. The device is illustrated schematically in FIG.3, and is constructed in a manner similar to an image intensifier tube,except that the silicon diode array is mounted in place of the phosphoroutput screen. In operation photoelectrons released by the photocathodeare accelerated by the cathode potential and focused on the silicondiode array. A gain of up to 6,000 can be achieved with thisarrangement, and photon noise is then predominant over other noise sothat photon noise limited detection is achieved.

FIG. 3 illustrates a suitable signal detection system for the shearinginterferometer. Each photodiode has its own channel, and each channelconsists of a pream plifier 60, a bandpass filter 62 tuned to themodulation frequency kHz in the disclosed embodiment), and a phasedetector 64. The outputs of the phase detectors are then combined in theanalog data processor as will be explained later. In the disclosedembodiment the bandpass filter has a bandwidth of about 1 kHz which willaccommodate the expected rate of change of wavefront phase, which may beseveral hundred Hz. The filter functions as an integrating network withits band width of 1 kHz effectively averaging the phase of the 10 kHzmodulating frequency over a period of one millisecond. The phasedetector compares the phase of the 10 kHz signal with the phase of areference signal and produces an analog output signal proportional tothe difference in phase.

Because of the cyclic nature of the modulation the phase values areobtained modulo 1 wavelength. This means that unambiguous phasemeasurements may be made only over a range of 1*: Va cycle relative tothe phase of the reference signal. This is one reason for using a closedloop system, as will be further explained later, because in such asystem the phase error is nulled, and the phase sensor need not operateover an unlimited range.

It is evident that all phase values produced by the interferometer arerelative. Thus, one of the phase values (most conveniently the centerone) must arbitrarily be designated as the reference and all other phasevalues are computed relative to the reference. In an optical imagingsystem the absolute phase of the lightwaves is irrelevant.

The computation required to convert the x and y phase values produced bythe shearing interferometer into the required phase correction values ismathematically a matrix inversion followed by a least squares smoothing.To carry out this operation with a digital computer for a large numberof channels at the required speed would require a relatively large, highspeed machine with input and output multiplexers and A/D and D/Ahardware. The accuracy requirements are relatively low (on the order of1%), and this fact coupled with the need for high speed operation withparallel input and output channels makes the use of a special purposeanalog computer a better choice.

The function and operation of the parallel data processor is illustratedin FIGS. 4, 5 and 6. Referring to FIGS. 4 and 5, it can be seen that anypoint in the illustrated phase corrector array, such as N, in FIG. 4 isconnected to surrounding points only through four measured phase shiftvectors a, b, c, and d which are al- 6 gebraically added to the phaseshifts existing at the points A, B, C, and D. The phase shift value at Nis therefore N=l/4(A+B+C+D+abc+d).

Referring to FIG. 5, there is depicted the interrelationship of phaseshift vectors for a complete matrix consisting of 32 measured phaseshift vectors (the outputs from the interferometer) for 21 points (witheach point representing a correction element in the phase corrector).The phase shift value for each separate point may be computed by aseparate analog computing circuit 68 for this function as shown in FIG.6. This results in an analog parallel data processor having twentycircuits for the 21 point matrix (one of the points, e.g., the centerone, is grounded as a reference). Each computing circuit has eightinputs and one output, although some of the points around the peripheryof the matrix do not utilize all of the inputs. Four of the inputs (a,b, c, d) are from the interferometer and are voltages proportional tomeasured phase shifts. The other four inputs (A, B, C, D) are voltageoutputs of other computing circuits. Eaclh input terminal has an inputresistor of equal value. The summing amplifier 68 effectively adds thetotal current from the top six terminals, substracts the current fromthe bottom two terminals, and produces an output proportional to theresult. As is well known in the art a resistor is utilized in a negativefeedback loop to control the gain of the amplifier. A fast settlingoperational amplifier, such as Burr- Brown 3500 CN, which has internalcompensation, is suitable for use in the circuit of FIG. 6. The outputof each computing circuit 68 is directed to an integrating amplifier 70,shown schematically as an amplifier with a capacitance in its feedbackloop. Typically, amplifier 70 would provide selectable gain and bandwithcapabilities to optimize system operational stability. An integratingamplifier is utilized in the preferred closed-loop embodiment becausephase excursions over the entire wavefront may be as much as severalwavelengths, but the output of each phase detector channel from theinterferometer provides unambiguous wavefront information up to only i/a wavelength. The integration by amplifier 70 allows an analog voltageto be built up and stored over several measurement cycles, which thencauses the mirror 20 to deform by several wavelengths. The output fromintegrating amplifier 70 is received by a high voltage driver amplifier72. Each driver amplifier drives one piezoelectric element of thewavefront corrector. This amplifier accepts a low level voltage signalat its input, and converts it to a voltage of up to several thousandvolts to drive the piezoelectric elements of the phase corrector.

An alternative embodiment for the analog parallel data processorcomprises a conduction matrix rather than a voltage matrix as shown. inFIG. 6. One advantage of a conduction matrix over a voltage matrix isthat its design is considerably simpler. The matrix is similar to thematrix illustrated in FIG. 5, and includes twenty current generators(one for every point in the matrix but the center point). Adjacentpoints in the matrix are connected by identical resistors. Al] x and yinputs to the current generators are voltage outputs from theinterferometer and are proportional to the measured phase differences.The output of each current generator is applied to one node of thearray. One current generator 78 and its connection to the array isillustrated in FIG. 7. x and y inputs from the shearing interferometerare received as inputs b, c, a, and d to an amplifier 80. The amplifierhas both negative and positive feedback. For an amplifier connected asillustrated in FIG. 7 and wherein all the resistors R are equal, theoutput current 1 l/R (a b c d). The voltage v at the output of thecurrent generator 1 X R, which 1(a b c d). The voltage V is dependentupon both the current 1 produced by the current generator 78 and thecombined voltage from adjacent nodes to which it is connected in thematrix by identical resistors. The combined voltage from adjacent nodesis the average of the voltages A, B, C and D or A (A+ B C+ D).Therefore, the total output voltage V0 A (A+B-l-C+D+abc+d), which is thedesired voltage.

The system has been described thus far as a closed loop system in whichthe reference image for the phase distortion detector is picked offafter the wavefront corrector has operated on the image. In an open loopdesign the reference image for the phase distortion detector is pickedoff before the wavefront is incident on the phase corrector. A closedloop system has several very important advantages. Most important, sincethe reference image is picked off after the wavefront has travelledthrough the optical system and phase corrector, in principle the systemwill compensate for any distortion introduced by the optical system upto the point where the imaging and reference beams are separated, sothat near diffraction limited performance may be obtained from somewhatimperfect optical systems. A closed loop system also minimizes the phaseerror without the need for critical calibration or adjustment ofcomponents within the loop. A further advantage of a closed loop systememploying an integrator (like element 70), is that the range of appliedphase correction is not limited by the instantaneous measuringcapability of the phase error sensing system. This is very desirable ina system (as described in detail in this patent application) in whichthe required phase correction may be several wavelengths, and the phaseerror sensing system has a more limited range (1 wavelength). However,embodiments of the present invention might be desired under somecircumstances based on an open loop design.

Several considerations should be borne in mind to optimize the designsof the shearing interferometer and the phase corrector. Reference shouldbe made to FIG. 8 for consideration of other parameters. .FIG. 8 shows awavefront defined as d) (x) displaced by a shear distance s from asheared wavefront. The distance over which each waveform is sampled (thewidth of the photodetector) is defined as a. The distance over which thewavefront is sampled is centered a distance d from the origin. p is thedistance between adjacent phase corrector elements in the phasecorrector. An analysis of the various parameters in the interferometerand phase corrector (which will not be detailed herein) has indicatedthat interferometer error is minimzed when the phase sampling area isapproximately centered, as shown in FIG. 8B, with respect to the gridpoints at half the grid spacing p plus half the shear distance s. Thiscondition is intuitively satisfying because at this location the phasesampling area covers the maximum common area of the two shearedwavefronts. Also in the special case where the shear distance s is equalto the grid spacing p, it accurately describes the correct samplingposition which is coincident with the grid.

When the wavefront being measured possesses phase excursions of severalwavelengths, as is the case with atmospheric distortion, it may benecessary to use a small shear distance s in order to avoid themeasurement ambiguity that would occur if the phase measured by thephase error sensing system exceeds one-half wavelength. This isillustrated in FIGS. 8A and 8B which shows a shearing interferometerwherein the shear distance s is less than the spacing p between adjacentgrid points. In most shearing interferometers of the prior art, theshear distance sis typically selected to be equal to the spacing p. Byselecting s to be a fraction of p according to the formula l/n p, theinterferometer may unambiguously measure phase differences existingbetween adjacent grid points which are greater than AM. If, for example,n in the above formula is selected to be four, then phase excursions ofup to two wavelengths between adjacent grid points may be measuredwithout exceeding the i /2)\ range of the phase detectors.

The number of phase corrector elements in the disclosed embodiment is 21which is a suitable number for a telescope of up to about 0.5 meteraperture. For larger telescopes the number of phase corrector elementsmay be several hundred.

Referring to FIG. 9, there is illustrated another preferred embodimentof the system which is similar to the embodiment of FIG. 1 except thatthe phase corrector system is different. In the embodiment of FIG. 9,the phase of various points in the wavefront is changed by passing thelight beam twice through a crystal, the index of refraction of which isselectively varied in different areas of the crystal. A system which maybe made to operate in this manner is discussed in detail in theDecember, 1972 issue of Applied Optics, and particularly on page 2763thereof. Operation of the Pockels Readout Optical Memory (PROM) systemis based on the control of the refractive index of abismuth-silicon-oxide (Bi siO crystal as a function of the point bypoint light intensity of an image impressed on the crystal. When used asa phase corrector, the polarization of the applied light beam is alignedwith one of the induced birefringent axes so that the phase retardationat each point in the crystal is proportional to the exposure (lightintensity x time) received at that point. The point by point lightintensity impressed on the crystal is controlled by a cathode ray tubeadjacent to the crystal. The scanned pattern on the face of the CRT iscontrolled in accordance with the output of the phase distortiondetector system.

In yet another embodiment, not illustrated, the refractive index of abismuth-silicon-oxide crystal is varied as a function of applied voltageacross the crystal. In one embodiment the crystal is about onemillimeter thick and fifteen millimeters in diameter. A singletransparent electrode is placed on the front surface, and an array of 21directly addressed electrodes are positioned on the back reflectingsurface. The crystal phase corrector is a solid state device, and aresponse time of less than one millisecond should be obtainable.

Although the disclosed embodiment is a system for eliminating distortionfrom a plane wavefront, it should be realized that the teachings of thisinvention might be utilized to achieve a wavefront having any desiredshape such as a spherical or elliptical wavefront.

While several embodiments have been described, the teachings of thisinvention will suggest many other embodiments to those skilled in theart.

I claim:

1. In an optical system for imaging objects of interest through theatmosphere, apparatus for the real-time detection and correction ofphase distortions in a wavefront being imaged by said optical system,said phase distortions being at least in part produced by turbulencewithin the atmosphere through which said object of interest is imaged,said apparatus comprising:

a. lateral shearing interferometer means, having said distortedwavefront incident thereon, for simultaneously determining in paralleland in real time relative phase differences between different areas ofsaid distorted wavefront, and for generating signals indicative of saidphase differences, said lateral shearing interferometer means includingmeans for permitting said interferometer means to operate with whitelight;

b. means responsive to said phase difference signals for simultaneouslygenerating in parallel and in real time phase corrector signalsindicative of phase corrections for different areas of said distortedwavefront to achieve a corrected wavefront; and,

0. phase corrector means having said distorted wavefront incidentthereon and being responsive to said phase corrector signals forchanging in real time phase differences between different areas of saiddistorted wavefront for changing the shape of said distorted wavefrontto achieve said corrected wavefront.

2. A system as set forth in claim 1 and wherein the system is a closedloop system wherein said phase corrector means changes the shape of thewavefront before the wavefront is incident upon said lateral shearinginterferometer means for determining phase differences.

3. A system as set forth in claim 1 wherein said phase corrector meansincludes a mirror means having an array of electroresponsive elements,with each element being responsive to one of said phase correctorsignals, for selectively deforming the mirror to change the shape of thewavefront.

4. A system as set forth in claim 1 wherein said phase corrector meansincludes a refractive phase correction means for selectively changingits index of refraction in different areas upon which the wavefront isincident to change the shape of the wavefront.

5. A system as set forth in claim 1 wherein said shearing interferometermeans includes:

a. first and second diffraction gratings, each of which has a slightlydifferent grating frequency, for shearing the wavefront in a firstdirection;

b. a first achromatizing diffraction grating following said first andsecond gratings and having a grating frequency substantially halfwaybetween the grat ing frequencies of said first and second diffractiongratings, to achromatize the interferometer to allow it to operate withwhite light;

0. a first field lens means following said achromatizing grating forforming a first sheared image of the entrance pupil of the opticalsystem at an image plane;

d. a first detector means positioned in said image plane of said firstfield lens means for detecting the shearing interferogram formedthereat;

e. third and fourth diffraction gratings, each of which has a slightlydifferent grating frequency, for shearing the wavefront in a seconddirection substantially perpendicular to said first direction;

f. a second achromatizing diffraction grating, following said third andfourth gratings and having a grating frequency substantially halfwaybetween the grating frequencies of said third and fourth diffrac- 10tion gratings, to achromatize the interferometer to allow it to operatewith whitelight;

g. a second field .lens means followingsaid second achromatizing,grating for forming a second sheared image of the entrance pupil of theoptical system at an image plane; and

h. a second detector means positioned in saidimage plane of said secondfield lens means for detecting the shearing interferogram formedthereat.

6. A system as set forth in claim 5 and wherein:

a. said first and second detector means each includes an array ofphotodetectors;

b. said phase corrector means includes an array of electroresponsiveelements, each of which is responsive to one of said-phase correctorsignals and each of which is separated. from adjacent electroresponsiveelements by a distance p;

c. said lateral shearing interferometer means shears the wavefront by ashear distance s in said first direction and said second direction; and

d. the detectors in said first and second detector arrays areapproximately centered with respect to the electro-responsive elementsin said phase corrector array at half the distance p plus half the sheardistance s.

7. A system as set forth in calim 6 and including translating drivemeans for moving said first, second, third and fourth diffractiongratings to heterodyne the shearing interferograms detected by saidfirst and second detector means.

8. A system as set forth in claim 1 and wherein the system is utilizedin a telescope having an entrance pupil and a prime focal plane and thesystem includes means for relaying an image of the wavefront incident onthe telescope entrance pupil onto said phase corrector means and forrelaying the image formed at said prime focal plane onto said lateralshearing interferometer means.

9. A system as set forth in claim 8 wherein the system is a closed loopsystem wherein said relaying means relays the image onto said means fordetermining phase differences after the wavefront has been incident onsaid phase corrector means.

10. A system as set forth in claim 9 wherein said shearinginterferometer means includes:

a. first and second diffraction gratings, each of which has a slightlydifferent grating frequency, for shearing the wavefront in a first.direction;

b. a first achromatizing diffraction grating following said first andsecond gratings and having a grating frequency substantially halfwaybetween the grating frequencies of said first and second diffractiongratings, to achromatize the interferometer to allow it to operate withwhite light;

c. a first field lens means following said achromatizing grating forforming a first sheared image of the entrance pupil of the telescope atan image plane;

d. a first detector means positioned in said image plane of said firstfield lens means for detecting the shearing interferogram formedthereat;

e. third and fourth diffraction gratings, each of which has a slightlydifferent grating frequency, for shearing the wavefront in a seconddirection substantially perpendicular to said first direction;

f. a second achromatizing diffraction grating, following said third andfourth gratings and having a grating frequency substantially halfwaybetween the grating frequencies of said third and fourth diffrac- 1 1tion gratings, to achromatize the interferometer to allow it to operatewith white light;

g. a second field lens means following said second achromatizing gratingfor fonning a second sheared image of the entrance pupil of thetelescope at an image plane; and

h. a second detector means positioned in said image plane of said secondfield lens means for detecting the shearing interferogram formedthereat.

11. A system as set forth in claim wherein:

a. said first and second detector means each includes an array ofphotodetectors;

b. said phase corrector means includes an array of electroresponsiveelements, each of which is responsive to one of said phase correctorsignals and translating drive means for moving said first, second, thirdand fourth diffraction gratings to heterodyne the shearinginterferograms detected by said first and second detector means.

1. In an optical system for imaging objects of interest through theatmosphere, apparatus for the real-time detection and correction ofphase distortions in a wavefront being imaged by said optical system,said phase distortions being at least in part produced by turbulencewithin the atmosphere through which said object of interest is imaged,said apparatus comprising: a. lateral shearing interferometer means,having said distorted wavefront incident thereon, for simultaneouslydetermining in parallel and in real time relative phase differencesbetween different areas oF said distorted wavefront, and for generatingsignals indicative of said phase differences, said lateral shearinginterferometer means including means for permitting said interferometermeans to operate with white light; b. means responsive to said phasedifference signals for simultaneously generating in parallel and in realtime phase corrector signals indicative of phase corrections fordifferent areas of said distorted wavefront to achieve a correctedwavefront; and, c. phase corrector means having said distorted wavefrontincident thereon and being responsive to said phase corrector signalsfor changing in real time phase differences between different areas ofsaid distorted wavefront for changing the shape of said distortedwavefront to achieve said corrected wavefront.
 2. A system as set forthin claim 1 and wherein the system is a closed loop system wherein saidphase corrector means changes the shape of the wavefront before thewavefront is incident upon said lateral shearing interferometer meansfor determining phase differences.
 3. A system as set forth in claim 1wherein said phase corrector means includes a mirror means having anarray of electroresponsive elements, with each element being responsiveto one of said phase corrector signals, for selectively deforming themirror to change the shape of the wavefront.
 4. A system as set forth inclaim 1 wherein said phase corrector means includes a refractive phasecorrection means for selectively changing its index of refraction indifferent areas upon which the wavefront is incident to change the shapeof the wavefront.
 5. A system as set forth in claim 1 wherein saidshearing interferometer means includes: a. first and second diffractiongratings, each of which has a slightly different grating frequency, forshearing the wavefront in a first direction; b. a first achromatizingdiffraction grating following said first and second gratings and havinga grating frequency substantially halfway between the gratingfrequencies of said first and second diffraction gratings, toachromatize the interferometer to allow it to operate with white light;c. a first field lens means following said achromatizing grating forforming a first sheared image of the entrance pupil of the opticalsystem at an image plane; d. a first detector means positioned in saidimage plane of said first field lens means for detecting the shearinginterferogram formed thereat; e. third and fourth diffraction gratings,each of which has a slightly different grating frequency, for shearingthe wavefront in a second direction substantially perpendicular to saidfirst direction; f. a second achromatizing diffraction grating,following said third and fourth gratings and having a grating frequencysubstantially halfway between the grating frequencies of said third andfourth diffraction gratings, to achromatize the interferometer to allowit to operate with white light; g. a second field lens means followingsaid second achromatizing grating for forming a second sheared image ofthe entrance pupil of the optical system at an image plane; and h. asecond detector means positioned in said image plane of said secondfield lens means for detecting the shearing interferogram formedthereat.
 6. A system as set forth in claim 5 and wherein: a. said firstand second detector means each includes an array of photodetectors; b.said phase corrector means includes an array of electroresponsiveelements, each of which is responsive to one of said phase correctorsignals and each of which is separated from adjacent electroresponsiveelements by a distance p; c. said lateral shearing interferometer meansshears the wavefront by a shear distance s in said first direction andsaid second direction; and d. the detectors in said first and seconddetector arrays are approximately centered with respect to theelectro-responsive elements in said phase corrector array at half thEdistance p plus half the shear distance s.
 7. A system as set forth incalim 6 and including translating drive means for moving said first,second, third and fourth diffraction gratings to heterodyne the shearinginterferograms detected by said first and second detector means.
 8. Asystem as set forth in claim 1 and wherein the system is utilized in atelescope having an entrance pupil and a prime focal plane and thesystem includes means for relaying an image of the wavefront incident onthe telescope entrance pupil onto said phase corrector means and forrelaying the image formed at said prime focal plane onto said lateralshearing interferometer means.
 9. A system as set forth in claim 8wherein the system is a closed loop system wherein said relaying meansrelays the image onto said means for determining phase differences afterthe wavefront has been incident on said phase corrector means.
 10. Asystem as set forth in claim 9 wherein said shearing interferometermeans includes: a. first and second diffraction gratings, each of whichhas a slightly different grating frequency, for shearing the wavefrontin a first direction; b. a first achromatizing diffraction gratingfollowing said first and second gratings and having a grating frequencysubstantially halfway between the grating frequencies of said first andsecond diffraction gratings, to achromatize the interferometer to allowit to operate with white light; c. a first field lens means followingsaid achromatizing grating for forming a first sheared image of theentrance pupil of the telescope at an image plane; d. a first detectormeans positioned in said image plane of said first field lens means fordetecting the shearing interferogram formed thereat; e. third and fourthdiffraction gratings, each of which has a slightly different gratingfrequency, for shearing the wavefront in a second directionsubstantially perpendicular to said first direction; f. a secondachromatizing diffraction grating, following said third and fourthgratings and having a grating frequency substantially halfway betweenthe grating frequencies of said third and fourth diffraction gratings,to achromatize the interferometer to allow it to operate with whitelight; g. a second field lens means following said second achromatizinggrating for forming a second sheared image of the entrance pupil of thetelescope at an image plane; and h. a second detector means positionedin said image plane of said second field lens means for detecting theshearing interferogram formed thereat.
 11. A system as set forth inclaim 10 wherein: a. said first and second detector means each includesan array of photodetectors; b. said phase corrector means includes anarray of electroresponsive elements, each of which is responsive to oneof said phase corrector signals and each of which is separated fromadjacent electroresponsive elements by a distance p; c. said lateralshearing interferometer means shears the wavefront by a shear distance sin said first direction and said second direction; and d. the detectorsin said first and second detector arrays are approximately centered withrespect to the electro-responsive elements in said phase corrector arrayat half the distance p plus half the shear distance s.
 12. A system asset forth in claim 11 and including translating drive means for movingsaid first, second, third and fourth diffraction gratings to heterodynethe shearing interferograms detected by said first and second detectormeans.